The long-term goal of this application has been to understand how autoreactive B cell responses are prevented in healthy individuals and how they are initiated and sustained in autoimmune disease. Specifically, B cells that produce anti-dsDNA antibodies (Abs) are studied. These Abs are highly significant clinically;they are one of the diagnostic criteria of the autoimmune disease Systemic Lupus Erythematosus (SLE), and they have been implicated in mediating tissue pathology. The approach has been to develop a transgenic (Tg) model using a heavy chain-only Tg (VH3H9) that can pair with endogenous light chains to generate both anti-DNA and non-DNA Abs. Here, the development of anti-dsDNA B cells can be tracked in the context of a diverse B cell repertoire in non-autoimmune and autoimmune-prone backgrounds. Taking this strategy, anti-dsDNA Abs are undetectable in the serum of BALB/c mice, yet the autoreactive B cells persist in the bone marrow and periphery with a phenotype indicative of antigen-mediated developmental arrest. In contrast, in autoimmune animals they are mature and by ten weeks of age autoAbs are detectable in the serum. Specific aims of this competitive renewal are to compare and contrast the differentiation and activation potential of anti-dsDNA B cells from non-autoimmune and autoimmune mice. The impact of signal strength on B cell differentiation pathways will be investigated using low avidity naive, TH1 and TH2 effectors. The ability of bystander T cell activation to influence anti-dsDNA B cell fate will be explored. Having demonstrated that T regulatory cells can suppress the ability of T cell help to drive autoAb production in vivo, the mechanism behind this suppression will be investigated. The extent of anti-dsDNA B cell activation and differentiation following T-independent stimulation will also be assessed both in vitro and in vivo. Specifically, the functional capabilities of B cells exposed to TLR4 and TLR9 ligands will be investigated. It is hoped that through a better understanding of autoreactive B cell activation thresholds, and the differentiation pathways that ensue as a consequence of this activation, that more rational therapeutic strategies for treating autoimmune diseases will be developed.